\input pstfig
$
\psset{linewidth=0.8pt,labelsep=0.5ex,unit=2em}
\pnode(0,0){A}
\pnode(6,0){B}
\pnode(2,  3){D}
\pnode(5.5,3){C}
\pnode(2, 0){H}
\pcline(A)(B)
\Bput{a_2}
\pcline(B)(C)
\pcline(C)(D)
\Bput{a_1}
\pcline(D)(A)
\psset{linewidth=0.4pt}
\pcline[linestyle=dashed](A)(C)
\pcline[linestyle=dashed](B)(D)
\pcline[linestyle=dashed](D)(H)
\Bput{h}
\psIntersectionPoint(A)(C)(B)(D){P}
%\setLCNode(A){1}(P){-1}{PA}
%\setLCNode(B){1}(P){-1}{PB}
\setLDNode(P)(A){0.4}{Pma}
\setLDNode(P)(B){0.4}{Pmb}
%\psarc{<->}(P){0.4}{(PA)}{(PB)}
\pcarc[arcangleA=-30,arcangleB=-30]{<->}(Pma)(Pmb)
\Bput{\varphi}
$
\bye

